Business and consumers use a wide array of wireless devices, including cell phones, wireless local area network (LAN) cards, global positioning system (GPS) devices, electronic organizers equipped with wireless modems, and the like. The increased demand for wireless communication devices has created a corresponding demand for technical improvements to such devices. Generally speaking, more and more of the components of conventional radio receivers and transmitters are being fabricated in a single integrated circuit (IC) package. In order to simplify single chip designs and to make each design suitable for as many applications as possible, much emphasis has been placed on developing direct conversion receivers and software-defined radios.
In a direct-conversion radio, the information demodulation requires a down-conversion mixer to recover the transmitted baseband signal. The output of a down-conversion mixer is given by the equation:cos(RF)*cos(LO)=0.5[cos(RF−LO)+cos (RF+LO)]  (EQN. 1)where cos(RF) is the signal received through the antenna and cos(LO) is the reference clock signal from a local oscillator. The RF signal can be expanded to show components of modulation (i.e., RF=(ωcarrier+ωmodulation) t for a frequency modulated signal. Also the frequency of cos(LO) is close to or identical to the center frequency of RF (or ωcarrier). A square wave clock is an implementation option.
Multiplying the cos(RF) and cos(LO) signal produces two output signals: cos (RF−LO) and cos(RF+LO). The cos(RF−LO) signal is the recovered transmitted baseband signal (or ωmodulation). The cos(RF+LO) signal is a by-product not contributing to the recovered baseband signal and should be removed by low-pass filters in the baseband filter block.
FIG. 2 illustrates conventional radio frequency (RF) mixing stage 225 containing down-conversion mixer 250 according to an exemplary embodiment of the prior art. Down-conversion mixer 250 receives an input RF signal, cos(RF), from the receiver front-end amplifiers and a local oscillator reference signal, cos(LO), from a local oscillator and outputs a baseband signal to DC cancellation block 255. Down-conversion mixer 250, due to mismatches in its internal signal paths, develops a DC-offset at its output. DC-offset cancellation block 255 is required to remove the offset before connecting the mixer output to the baseband filters and automatic gain control (AGC) amplifier block 230. The baseband filters comprise cascading lowpass filters which provide a channel selectivity function. The AGC amplifiers provide programmable gain stages to condition the demodulated signal level.
When building high sensitivity direct-conversion radios (i.e., <−100 dBm) with good blocking characteristics, this architecture is impractical because the demodulated signal level at the mixer output is relatively small in comparison to the magnitude of the DC-offset and low frequency noise generated from these blocks.
Therefore, there is a need in the art for improved RF receivers that are low cost, high performance and easily integrated into a single integrated circuit chip. In particular, there is a need for an improved high sensitivity direct-conversion radios that do not require DC-offset cancellation circuits and complex baseband filters to remove noise.